Free NCERT Class 12 Chemistry Chapter 11 Notes Alcohols, Phenols and Ethers - Download PDF

NCERT Notes for Class 12 Chapter 7: Download PDF

These concise NCERT notes cover all the key concepts of alcohol, phenol and ethers to help students in quick revision before the exam. You can download the NCERT notes PDF from the button given below

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NCERT Notes for Class 12 Chapter 7

Alcohols, Phenols and Ethers notes are given below and they covers all concepts of the NCERT textbook and explains them in a concise and easy-to-understand manner. Whether revising or studying for the first time these NCERT notes are a boon for students who aim to score well in their exams.

Classification

The classification of compounds Alcohold, Phenols and Ethers makes their study systematic and hence simpler.

1. Alcohols Mono, Di, Tri or Polyhydric alcohols

Alcohols and phenols are classed as monohydric, dihydric, trihydric, or polyhydric depending on how many hydroxyl groups they have in their molecules.

According to the hybirdisation of Carbon atom to which the hydroxyl group is attached, Monohydric alcohols are further classified:

(i). Compounds containing $\mathrm{C}_{s p^3}-\mathrm{OH}$ group: Alcohols in which the –OH group is attached to an sp³ hybridised carbon atom of an alkyl group.

They are further classified as:

(a). Primary, secondary and tertiary alcohols: The –OH group is attached to primary, secondary and tertiary carbon atom.

Primary alcohol

Secondary alcohol

Tertiary alcohol

Primary Secondary Tertiary

(b). Allylic alcohols: The OH group is attached to a sp³ hybridised carbon adjacent to the carbon-carbon double bond, that is to an allylic carbon.

(c). Benzylic alcohols: The OH group is attached to a sp³ hybridised carbon atom next to an aromatic ring.

(ii) Compounds containing $\mathrm{C}_{s p^2}-\mathrm{OH}$ bond

These alcohols contain OH group bonded to a carbon-carbon double bond, i.e., to a vinylic carbon or to an aryl carbon. These alcohols are also known as vinylic alcohols.

Vinylic alcohol: $\mathrm{CH}_2=\mathrm{CH}-\mathrm{OH}$

2. Phenols Mono, Di and trihydric phenols

3. Ethers

Ethers are classified as simple or symmetrical, if the alkyl or aryl groups attached to the oxygen atom are the same, and mixed or unsymmetrical, if the two groups are different.

Nomenclature

1. Alcohols

The common name of an alcohol is derived from the common name of the alkyl group and adding the word alcohol to it. For example, CH₃OH is methyl alcohol.

IUPAC Nomenclature:

• Select the longest chain containing the OH group.

• Number the carbon so that the OH group gets the lowest possible position

• Remove e from the alkane name and add ol.

• Mention the position of the OH group

2. Phenols

The simplest hydroxy derivative of benzene is phenol. It is its common name and also an accepted IUPAC name.

IUPAC Nomenclature

• The parent compound is phenol.
• Number the ring in such a way so that the substituents gets the lowest possible position.
• Name and prefix substituents with their positions.

3. Ethers

Common names of ethers are derived from the names of alkyl/ aryl groups written as separate words in alphabetical order and adding the word ‘ether’ at the end.

IUPAC Nomenclature

• Name the longerst alkyl group as the base.
• Name the shorter alkyl group as an alkoxy group.
• Combine them as alkoxyalkane.

Structure of Functional Groups

Alcohols have sp³ hybridized oxygen atoms and hybrid atomic orbitals in a tetrahedral configuration. The R group determines the value of the ROH bond angle. Due to lone pair repulsion, this angle for methyl alcohol is (C – O – H) 108.9°.The -OH group in phenols is connected to sp² hybridized carbon, giving the C – O bond a partial double bond nature.

Alcohols and Phenols

Preparation of Alcohols

Alcohols are prepared by the following methods:

1. From alkenes

(i). By acid catalysed hydration

This reaction is in accordance with Markonikov's rule,

CH₃CH=CH₂ + H₂O → CH₃-CH(OH) - CH₃

Mechanism

It involves 3 steps:

Step1. Protonation of alkene to form carbocation by electrophilic attack of H₃O⁺

$\mathrm{H}_2 \mathrm{O}+\mathrm{H}^{+} \rightarrow \mathrm{H}_3 \mathrm{O}^{+}$

Step 2. Nucleophilic attack of water on carbocation

Step 3: Deprotonation to form an alcohol

(ii). By hydroboration-oxidation

By hydroboration-oxidation of alkenes, alcohols can be prepared, where BH₃ adds across the double bond followed by oxidation with H₂O₂/NaOH to give alcohols with anti-Markovnikov orientation.

2. From carbonyl compounds

(i). By reduction of aldehydes and ketones

RCHO + H₂ → RCH₂OH

$\mathrm{RCOR} \xrightarrow{\mathrm{NaBH} 4} \mathrm{R}-\mathrm{CH}(\mathrm{OH})-\mathrm{R}$

(ii). By reduction of carboxylic acids

$\mathrm{RCOOH} \xrightarrow{\text { LiAl }_4, \mathrm{H}_2 \mathrm{O}} \mathrm{ROH}$

3. From Grignard reagents

All three types of monohydric alcohols can be prepared by the use of Grignard reagents. Grignard reagents form addition compounds by nucleophile attack with aldehydes and ketones which on hydrolysis with dilute acid yields alcohol.

$\mathrm{HCHO}+\mathrm{RMgX} \xrightarrow{\mathrm{H}_2 \mathrm{O}} \mathrm{RCH}_2 \mathrm{OH}+\mathrm{Mg}(\mathrm{OH}) \mathrm{X}$

$\mathrm{RCHO}+\mathrm{RMgX} \xrightarrow{\mathrm{H}_2 \mathrm{O}} \mathrm{R}-\mathrm{CH}(\mathrm{R}) \mathrm{OH}+\mathrm{Mg}(\mathrm{OH}) \mathrm{X}$

$\mathrm{RCOR}+\mathrm{RMgX} \xrightarrow{\mathrm{H}_2 \mathrm{O}} \mathrm{R}-\mathrm{C}(\mathrm{R})(\mathrm{R}) \mathrm{OH}+\mathrm{Mg}(\mathrm{OH}) \mathrm{X}$

Preparation of Phenols

Phenol, also known as carbolic acid, and is commercially produced synthetically. In the laboratory, phenols are prepared from benzene derivatives by any of the following methods:

1). From haloarenes

Chlorobenzene is fused with NaOH at 623K and 320 atmospheric pressure. Phenol is obtained by acidification of sodium phenoxide so produced. The reaction occurs as follows.

2). From benzenesulfonic acid

Benzene is sulphonated with oleum and benzene sulphonic acid so formed is converted to sodium phenoxide on heating with molten sodium hydroxide. Acidification of the sodium salt gives phenol. The reaction occurs as follows:

3). From diazonium salts

A diazonium salt is formed by treating an aromatic primary amine with nitrous acid (NaNO₂ + HCl) at 273-278 K. Diazonium salts are hydrolysed to phenols by warming with water or by treating with dilute acids. The reaction occurs as follows.

4). From Cumenes

Phenol is manufactured from the hydrocarbon, cumene. Cumene (isopropylbenzene) is oxidised in the presence of air to cumene hydroperoxide. It is converted to phenol and acetone by treating it with dilute acid. Acetone, a by-product of this reaction, is also obtained in large quantities by this method. The reaction occurs as follows:

Physical Properties

Alcohols and phenols consist of two parts, an alkyl/aryl group and a hydroxyl group. The properties of alcohols and phenols are chiefly due to the hydroxyl group. The nature of alkyl and aryl groups simply modify these properties:

(i). Boiling points of alcohols and phenols rise with increasing carbon atoms due to stronger van der Waals forces, but decrease with branching due to reduced surface area. They have higher boiling points than hydrocarbons, ethers, haloalkanes, and haloarenes of similar masses, mainly because of intermolecular hydrogen bonding. For example, despite similar molecular masses, ethanol boils much higher than propane, while methoxymethane shows an intermediate boiling point, reflecting the absence of hydrogen bonding in ethers.

(ii). Lower alcohols are colourless liquids, C₅–C₁₁ alcohols are oily liquids, and C₁₂ and higher alcohols are waxy solids. Alcohols are miscible with water because their hydroxyl groups can form H-bonds with water. With increasing molecular mass, solubility decreases. Because polar molecules have intermolecular hydrogen bonding, the boiling points of alkanes are greater than expected.

(iii). These are colourless liquids or crystalline solids that turn coloured over time due to gradual oxidation in the presence of air. Carboxylic acid is another name for phenol. Phenols establish intermolecular H-bonds with other phenol molecules and with water due to the presence of a polar -OH bond.

(iv). Alcohols and phenols are soluble in water due to hydrogen bonding with water molecules. However, solubility decreases as the size of the hydrophobic alkyl or aryl group increases. Lower molecular mass alcohols are completely miscible with water.

Chemical Reactions

Alcohols react both as nucleophiles and electrophiles. The bond between O–H is broken when alcohols react as nucleophiles. They are versatile compounds.

Alcohols as Nucleophiles

Alcohols as electrophiles

1). Reactions involving cleavage of O–H bond

Reactions involving cleavage of the O–H bond in alcohols and phenols typically involve the release of a proton, showcasing their acidic nature. These reactions often form alkoxides or phenoxides when treated with active metals or strong bases.

i). Acidity of alcohols and phenols

a). Reaction with metals

$2 \mathrm{R}-\mathrm{OH}+2 \mathrm{Na} \xrightarrow{\text { yields }} 2 \mathrm{R}-\mathrm{O}-\mathrm{Na}+\mathrm{H}_2$

b). Acidity of alcohols

The acidic character of alcohols arises from the polar O–H bond. Electron-releasing groups increase electron density on the oxygen atom, reducing the O–H bond polarity and thus decreasing the acidity.

Alcohols are weaker acids than water.

b). Acidity of phenols

Because the phenoxide ion is stabilized through resonance, phenol is more acidic than alcohols. The presence of an electron withdrawing group raises phenol's acidity by stabilizing the phenoxide ion, whereas the presence of an electron releasing group lowers phenol's acidity by destabilizing the phenoxide ion. $s p^2$-hybridised carbon of the benzene ring directly bonded to the hydroxyl group is more electronegative than the oxygen itself, withdrawing electron density from the –OH group. This increases the polarity of the O–H bond, promoting ionisation and formation of the phenoxide ion.

(ii). Esterification

Reaction of Alcohols and phenols with carboxylic acids, acid chlorides and acid anhydrides form esters.

$\mathrm{Ar} / \mathrm{R}-\mathrm{OH}+\mathrm{R}-\mathrm{COOH} \xrightarrow{\mathrm{H}^{+}} \mathrm{Ar} / \mathrm{ROCOR}+\mathrm{H}_2 \mathrm{O}$

Ar/R-OH + (RCO,)₂O + H⁺↔ Ar/ROCOR, + RCOOH

R/ArOH + R,COCl Pyridine→ R/ArOCOR, + HCl

2). Reactions involving cleavage of carbon oxygen (C–O) bond in alcohols

Reactions involving cleavage of the C–O bond in alcohols typically occur during substitution or elimination, where the hydroxyl group is replaced or removed forming alkyl halides or alkenes.

i). Reaction with hydrogen halides

ROH + HX yields→ RX + H₂O

ii). Reaction with phosphorus trihalides:

$3 \mathrm{R}-\mathrm{OH}+\mathrm{PBr}_3 \rightarrow 3 \mathrm{R}-\mathrm{Br}+\mathrm{H}_3 \mathrm{PO}_3$

iii). Dehydration

Alcohols undergo dehydration means removal of a molecule of water to form alkenes on treating with a protic acid.

Order of reaction

Tertiary > Secondary > Primary

Mechanism of dehydration

Step 1: Formation of protonated alcohol

Step 2: Formation of carbocation

Step 3: Formation of ethene by elimination of a proton.

iv). Oxidation

Formation of a carbon oxygen double bond with cleavage of an O-H and C-H bonds is involved in oxidation of alcohol.

Alcohol, Acidified KMnO₄→ Carboxylic acid

RCH₂OH Oxidation→ RCHO yields→ RCOOH

3). Reactions of phenol

Phenol undergoes characteristic reactions. Its aromatic ring is highly reactive due to the activating effect of the –OH group.

i). Electrophilic aromatic substitution

Electrophilic substitution reactions take place on aromatic ring in phenols. The OH group attached to the benzene ring activates it towards electrophilic substitution. Also, it directs the incoming group to ortho and para positions in the ring as these positions become electron rich due to the resonance effect caused by OH group.

a). Halogenation

Anisole reacts with an alkyl halide and acyl halide introducing alkyl and acyl groups in ortho and para positions.

b). Nitration

With dilute nitric acid at low temperature (298 K), phenol yields a mixture of ortho and para nitrophenols. The reaction occurs as follows.

ii). Reimer-Tiemann reaction

Phenols for aromatic compounds containing EDG when refluxed with CHCl₃ and alkali yield o- and p- hydroxybenzaldehyde. The ortho product is the predominant product. It is an electrophilic substitution on PhO^- ion. The electrophile is dichlorocarbene (:CCl₂) which contains a C with only six electrons.

iii). Kolbe’s reaction

Phenol when heated at(390-410K) under pressure with CO₂ and alkali gives salicylic acid after acidification in addition to some amount of p-isomer.

iv). Reaction of phenol with zinc dust

When phenol is distilled with zinc dust, benzene is obtained. The reaction occurs as follows:

v). Oxidation

Some Commercially Important Alcohols

Two commercially important alchols are Methanol and ethanol.

Methanol (CH₃OH)

Wood-spirit is another name for it. It is a clear liquid with no discernible colour. It reaches a temperature of 337 degrees Fahrenheit when it boils. It is extremely poisonous. Even little doses can cause blindness, and excessive doses can be very fatal.

Paints, varnishes, and other products use it as a solvent.It can be used to make formaldehyde.

Ethanol (C₂H₅OH)

It is known as denatured spirit when combined with CuSO₄ and pyridine.It is a colourless liquid having boiling point 351 K.

$\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}+\mathrm{H}_2 \mathrm{O} \xrightarrow{\text { Invertase }} \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6$

$\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6 \xrightarrow{\text { Zymase }} 2 \mathrm{C}_2 \mathrm{H}_5 \mathrm{OH}+2 \mathrm{CO}_2$

It is a good solvent, Sterilization of surgical tools in laboratories and hospitals.

Ethers

Ethers are organic compounds in which two alkyl or aryl groups are bonded to the same oxygen atom.

Preparation of Ether

1). By dehydration of alcohols

Alcohols lose water in the presence of protic acids like H₂SO₄ or H₂PO₄. The product formed alkene or ether depends on temperature. For instance, ethanol gives ethene at 443 K, while at 413 K, it mainly forms ethoxyethane.

CH₃CH₂OH$\xrightarrow[443 \mathrm{~K}]{\mathrm{H}_2 \mathrm{SO}_4} \mathrm{CH}_2=\mathrm{CH}_2$

CH₃CH₂OH $\xrightarrow[413 \mathrm{~K}]{\mathrm{H}_2 \mathrm{SO}_4} \mathrm{C}_2 \mathrm{H}_5 \mathrm{OC}_2 \mathrm{H}_5$

2). Williamson synthesis

It is the best method to prepare all type of ethers, that is, simple, mixed or aromatic ethers. Here, alkyl halides are treated with sodium alkoxide in presence of magnesium to give ethers. It involves S_N2 mechanism during the attack of R-O^- on R-X, that is, backside attack occurs here. The reaction occurs as follows:

Some examples include:

Physical Properties of Ethers

Since ethers' C-O bonds are polar, they have a net dipole moment. Their boiling points are equivalent to alkanes with similar molecular weights, although they are lower than alcohols. It's because ethers don't have H-bonding. Ether miscibility with water is similar to that of alcohols of same molar mass. It's because ethers, like alcohols, can make H-bonds with water.

Chemical Reactions

Ethers are generally unreactive but undergo cleavage with strong acids like HI or HBr, forming alcohols and alkyl halides.

1). Cleavage of C–O bond in ethers

The cleavage of C-O bond in ethers takes place under drastic conditions with excess of hydrogen halides. The reaction of dialkyl ether gives two alkyl halide molecules

R-O-R + HX → RX + R-OH

R-OH + HX → R-X + H₂O

2). Electrophilic substitution reactions

The alkoxy group is ortho, para directing and activates the aromatic ring towards electrophilic substitution in the same way as in phenol.

i). Halogenation

ii). Friedel craft’s reaction

Anisole reacts with an alkyl halide and acyl halide introducing alkyl and acyl groups in ortho and para positions.

iii). Nitration

Anisole reacts with a mixture of concentrated sulphuric and nitric acids to yield a mixture of ortho and para nitroanisole. The reaction occurs as follows.

Alcohols ,Phenols and Ethers Previous Year Question and Answer

Slected questions from previous year exams are given below:

Question 1: Which of the following are benzylic alcohols?

(1) (i) and (ii)

(2) (iii) and (iv)

(3) (ii) and (iii)

(4) (ii) and (iv)

Answer:

The answer is the option (ii) and (iii).

The most prominent way to identify benzylic alcohol is to check if an -OH group is attached to a $s p^3$ hybridised carbon atom on an aromatic ring.

As in the case of option (ii) and (iii), the $s p^3$ hybridised carbon is attached to a benzene ring these are benzylic alcohols but in option (i) and (iv), the $s p^3$ hybridised carbon is not attached to a benzene ring.

Hence, the answer is the option (3).

Question: Phenol can be distinguished from ethanol by the reactions with _________.

(i) $B r_2 /$ water

(ii) $N a$

(iii) Neutral $\mathrm{FeCl}_3$

(iv) All the above

(1) (i) and (iii)

(2) (iii) and (iv)

(3) (ii) and (iii)

(4) None of above

Answer:

The answer is the option (i) and (iii).

Phenol can easily be distinguished from ethanol by adding bromine water as it results in a white ppt of tri-bromo phenol or Neutral FeCl₃ can also be used for this purpose. But ethanol being aliphatic alcohol will not react with either of the compounds.

Hence, the answer is the option (1). Question:

Question: Which of the following reactions will yield phenol?

(1) (A), (B) and (C)

(2) (A), (B) and (D)

(3) (A), (C) and (D)

(4) (B), (C) and (D)

Answer:

The answer is the option (A), (B), (C). To carry out these reactions, the conditions required should be easy to maintain. The reaction occurring in (D) is a nucleophilic substitution of chlorobenzene which is not feasible as it requires drastic temperature and pressure conditions.

The reaction given in (A) is the Dow process, (B) is aniline diazotisation and (C) option contains a reaction from benzene sulphonic acid. All these reactions are feasible due to manageable reaction conditions.

Hence, the answer is the option (1).

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